From: Luke 8:19-21
The True Kinsmen of Jesus
 Then His (Jesus') mother and His brethren came to Him, but they could not reach Him for the crowd.  And He was told, "Your mother and your brethren are standing outside, desiring to see You."  But He said to them, "My mother and my brethren are those who hear the word of God and do it."
19-21. These words of our Lord show us that fulfillment of the Will of God is more important than kinship and that, therefore, our Lady is more united to her Son by virtue of her perfect fulfillment of what God asked of her, than by the Holy Spirit's using her to make Christ's body (cf. notes on Matthew 12:48-50 and Mark 3:31-35).
[Notes on Matthew 12:48-50 states:
48-50. Jesus obviously loved His Mother and St. Joseph. He uses this episode to teach us that in His Kingdom human ties do not take precedence. In Luke 8:19 the same teaching is to be found. Jesus regards the person who does the will of His Heavenly Father as a member of His own family. Therefore, even though it means going against natural family feelings, a person should do just that when needs be in order to perform the mission the Father has entrusted to him (cf. Luke 2:49).
We can say that Jesus loved Mary more because of the bonds between them created by grace than because He was her son by natural generation: Mary's divine motherhood is the source of all our Lady's other prerogatives; but this very motherhood is, in its turn, the first and greatest of the graces with which Mary was endowed.]
[Notes on Mark 3:31-35 states:
31-35. In Aramaic, the language used by the Jews, the word "brethren" is a broad term indicating kinship: nephews, first cousins, and relatives in general are called `brethren' (for further explanation cf. note on Mark 6:1-3). "Jesus did not say this to disown His mother, but to show that she is worthy of honor not only account of having given birth to Jesus, but also because she has all the virtues" (Theoplylact, "Enarratio In Evangelium Marci, in loc.").
Therefore, the Church reminds us that the Blessed Virgin "in the course of her Son's preaching received the words whereby, in extolling a kingdom beyond the concerns and ties of flesh and blood, He declared blessed those who heard and kept the word of God as she was faithfully doing" (Vatican II, "Lumen Gentium", 58).
Our Lord, then, is also telling us that if we follow Him we will share His life more intimately than if were a member of His family. St. Thomas explains this by saying that Christ "had an eternal generation and a generation in time, and gave preference to the former. Those who do the will of the Father reach Him by Heavenly generation [...]. Everyone who does the will of the Father, that is to say, who obeys Him, is a brother or sister of Christ, because he is like Him who fulfilled the will of His Father. But he who not only obeys but converts others, begets Christ in them, and thus becomes like the Mother of Christ" ("Commentary on St. Matthew", 12, 49-50.)
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.